1. Field of the Invention
The present invention relates to a method for manufacturing a discharge tube suitable for flash device of a simple camera, such as a lens-fitted photo film unit, in particular, a method that can reduce the manufacture process to realize cost reduction.
2. Explanations of the Prior Arts
A flash discharge tube is used in a flash device of a camera. The flash discharge tube is ordinarily manufactured in the following steps. After an electrode lead is inserted into a glass bead, the glass bead with the electrode lead is fixed to one end of a glass tube. Then, rare gas, such as xenon, is put into the glass tube from the other end thereof. Finally, the other end of the glass tube is sealed by the glass bead with the electrode.
Generally, a flash discharge tube is required to have sufficient performance lifetime for emitting light about 5000 times. In order to achieve this, the conventional flash discharge tube uses tungsten with high-melting point as an electrode, and a hard glass with approximately the same thermal expansion rate as tungsten is used to cover the tungsten in a tight manner. Tungsten electrode lasts a long time, but tungsten is expensive. In addition, the tungsten electrode can not be soldered directly. Thus a nickel pin is required for welding, and this results in further increase in cost.
In order to ensure good adhesion with the glass bead, the electrode lead needs to be oxidized at least in a predetermined adhesion area to fix the glass beads. Moreover, in order not to affect the electrode function as well as to ensure solder welding, oxidation of the surface of the electrode lead must be prevented at both ends of the electrode leads except the predetermined adhesion area.
JPA No.8-236023 discloses a manufacturing method of an electrode lead that consists of tungsten and nickel. According to the above patent, when the two materials are welded in the air by joining over the same axle, the whole surface of the electrode lead is oxidized by the heat generated in welding. Then, the glass bead is inserted into the electrode lead such that the glass bead is in contact with the predetermined adhesion area of the electrode lead. Finally, a gas burner heats the glass bead in deoxidation gas atmosphere to adhere the glass bead to the electrode lead. As welding of the two materials and oxidation treatment are simultaneously conducted in the manufacturing process, the number of processes can be reduced in comparison with the conventional method in which welding and oxidation treatment are separately conducted. Additionally, oxidation treatment is easily conducted to the electrode lead that has uneven surface.
Despite the above advantages, the electrode lead must be subject to deoxidation treatment in the area excluding the predetermined adhesion area. As a result, the glass bead is adhered to the electrode lead in reducing gas or hydrogen atmosphere so as to prevent the surface of the electrode lead excluding the predetermined adhesion area from being oxidized. As other deoxidation process, it is possible to remove oxide films by acid washing.
In the electrode lead with different materials, oxygen in the air naturally reacts with the whole electrode lead upon welding. It is necessary to have deoxidation treatment over the surface of the electrode lead except the predetermined adhesion area. In conducting deoxidation treatment by reducing gas, for example, an equipment to generate reducing gas atmosphere is required. As a result, the manufacture cost increases.
Low cost is desirable for a lens-fitted photo film unit including a flash unit. However, incorporating the discharge tube with such expensive materials increases the cost of the lens-fitted photo film unit. Particularly, the lens-fitted photo film unit does not need such a durable flash discharge tube because it is recycled after its usage. It is sufficient for the electrode lead of the lens-fitted photo film unit to emit light at most around 150 times including recycling and testing upon manufacture. For the purpose of cost reduction, Japanese Utility Model Laid-Open Publication No. 7-18132 discloses a discharge tube that uses an electrode lead of a single material, such as Kovar metal (alloy of nickel, iron, and cobalt). However, the above oxidize treatment can not be applied to this type of electrode lead, because it does not require the welding process to reduce the electrode lead.